Red Blood Cells of a Transgenic Mouse Expressing High Levels of Human Hemoglobin S Exhibit Deoxy-stimulated Cation Flux

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and Na⁺ transport in RBCs from control mice (C57Bl/6J) and a transgenic (α^Hβ^S[β^MDD]) mouse line that expresses high levels of human α^H and β^S-chains and has a small percent dense cells but does not exhibit anemia. In transgenic mouse RBCs (n= 5) under oxygenated conditions, K⁺ efflux was 0.22 ± 0.01 mmol/L cell × min and Na⁺ influx was 0.17 ± 0.02 mmol/L cell × min. Both fluxes were stimulated by 10 min deoxygenation in transgenic but not in control mice. The deoxy-stimulated K⁺ efflux from transgenic mouse RBCs was about 55% inhibited by 5 nm charybdotoxin (CTX), a blocker of the calcium activated K⁺-channel. To compare the fluxes between human and mouse RBCs, we measured the area of mouse RBCs and normalized values to area per liter of cells. The deoxy-simulated CTX-sensitive K⁺ efflux was larger than the CTX-sensitive K⁺ efflux observed in RBCs from SS patients. These results suggest that in transgenic mice, deoxygenation increases cytosolic Ca²⁺ to levels which open Ca²⁺-activated K⁺ channels. The presence of these channels was confirmed in both control and transgenic mice by clamping intracellular Ca²⁺ at 10 μm with the ionophore A23187 and measuring Ca²⁺-activated K⁺ efflux. Both types of mouse had similar maximal rates of CTX-sensitive, Ca²⁺-activated K⁺ efflux that were similar to those in human SS cells. The capacity of the mouse red cell membrane to regulate cytosolic Ca²⁺ levels was examined by measurements of the maximal rate of calmodulin activated Ca²⁺-ATPase activity. This activity was 3-fold greater than that observed in human RBCs thus indicating that mouse RBC membranes have more capacity to regulate cytosolic Ca²⁺ levels.

In summary, transgenic mouse RBCs exhibit larger values of deoxy-stimulated K⁺ efflux and Na⁺ influx when compared to human SS cells. They have a similar Ca²⁺-activated K⁺ channel activity to human SS cells while expressing a very high Ca²⁺ pump activity. These properties may contribute to the smaller percent of very dense cells and to the lack of adult anemia in this animal model.